1. Field of the Invention
The invention relates to a vehicle steering system in which an operating member is not mechanically coupled to a steering mechanism and the steering mechanism is driven by a motor.
2. Discussion of Background
Japanese Patent Application Publication No. 2004-90784 (JP 2004-90784 A) describes a steer-by-wire system in which a steering wheel that serves as an operating member is not mechanically coupled to a steering mechanism, a steering angle of the steering wheel is detected by an angle sensor, and the driving force of a steered system motor (steering actuator) that Is controlled on the basis of an output from the angle sensor is transmitted to the steering mechanism.
In the steer-by-wire system, because there is no mechanical coupling between the steering wheel and the steering mechanism, it is possible to prevent upthrust of the steering wheel in the event of a collision of a vehicle, and it is also possible to simplify the configuration of the steering mechanism and reduce the weight of the steering mechanism. In addition, the flexibility in arrangement of the steering wheel increases, and, furthermore, operating means other than a steering wheel, for example, a lever, a pedal, or the like, may be employed.
In the steer-by-wire system, the steering wheel and the steering mechanism are not mechanically coupled to each other. Therefore, if a malfunction occurs in electrical devices such as the steered system motor and the angle sensor that detects the steering angle of the steering wheel, it is not possible to execute steering control. Therefore, there is suggested the following steer-by-wire system in which electrical devices such as steered system motors and angle sensors are provided. In the steer-by-wire system, each kind of electrical device is provided in plurality, and when a malfunction occurs in one of the electrical devices, another electrical device of the same kind with no malfunction is used instead of the malfunctioning electrical device, thereby avoiding the situation where steering is disabled. However, this configuration results in a cost increase because each kind of electrical device needs to be provided in plurality.